[mlir] [LLVM IR] Introduce VaArgOp (#109260)

I find there is no LLVMOp corresponding to LLVM's [va_arg
instruction](https://llvm.org/docs/LangRef.html#va-arg-instruction) so I
tried to add one. This is helpful for clangir
(llvm/clangir#865).

New to MLIR and not sure who are the appropriate reviewers. Appreciated
in ahead for reviewing and triaging.

Author: ChuanqiXu9

mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td

@@ -582,6 +582,26 @@ def LLVM_LandingpadOp : LLVM_Op<"landingpad"> {
   let hasVerifier = 1;
 }
 
+//===----------------------------------------------------------------------===//
+// VAArgOp
+//===----------------------------------------------------------------------===//
+
+def LLVM_VaArgOp : LLVM_Op<"va_arg"> {
+  let arguments = (ins LLVM_AnyPointer:$arg);
+  let results = (outs LLVM_Type:$res);
+
+  let builders = [LLVM_OneResultOpBuilder];
+
+  let llvmBuilder = " $res = builder.CreateVAArg($arg, $_resultType); ";
+  let assemblyFormat = "$arg attr-dict `:` functional-type($arg, $res)";
+
+  string llvmInstName = "VAArg";
+  string mlirBuilder = [{
+    $res = $_builder.create<mlir::LLVM::VaArgOp>(
+      $_location, $_resultType, $arg);
+  }];
+}
+
 //===----------------------------------------------------------------------===//
 // CallOp
 //===----------------------------------------------------------------------===//

mlir/test/Dialect/LLVMIR/roundtrip.mlir

@@ -644,6 +644,9 @@ llvm.func @vararg_func(%arg0: i32, ...) {
   %list2 = llvm.alloca %1 x !llvm.struct<"struct.va_list_opaque", (ptr)> : (i32) -> !llvm.ptr
   llvm.intr.vacopy %list to %list2 : !llvm.ptr, !llvm.ptr
 
+  // CHECK: %[[RET:.+]] = llvm.va_arg %[[LIST2]] : (!llvm.ptr) -> i32
+  %ret = llvm.va_arg %list2 : (!llvm.ptr) -> i32
+
   // CHECK: llvm.intr.vaend %[[LIST]] : !llvm.ptr{{$}}
   // CHECK: llvm.intr.vaend %[[LIST2]] : !llvm.ptr{{$}}
   llvm.intr.vaend %list : !llvm.ptr

mlir/test/Target/LLVMIR/Import/basic.ll

@@ -76,8 +76,8 @@ declare void @llvm.va_start.p0(ptr)
 declare void @llvm.va_copy.p0(ptr, ptr)
 declare void @llvm.va_end.p0(ptr)
 
-; CHECK-LABEL: llvm.func @variadic_function
-define void @variadic_function(i32 %X, ...) {
+; CHECK-LABEL: llvm.func @variadic_function(%arg0: i32, ...) -> i32
+define i32 @variadic_function(i32 %X, ...) {
   ; CHECK: %[[ALLOCA0:.+]] = llvm.alloca %{{.*}} x !llvm.struct<"struct.va_list", (ptr)> {{.*}} : (i32) -> !llvm.ptr
   %ap = alloca %struct.va_list
   ; CHECK: llvm.intr.vastart %[[ALLOCA0]]
@@ -87,11 +87,13 @@ define void @variadic_function(i32 %X, ...) {
   %aq = alloca ptr
   ; CHECK: llvm.intr.vacopy %[[ALLOCA0]] to %[[ALLOCA1]]
   call void @llvm.va_copy.p0(ptr %aq, ptr %ap)
+
+  %ret = va_arg ptr %aq, i32
   ; CHECK: llvm.intr.vaend %[[ALLOCA1]]
   call void @llvm.va_end.p0(ptr %aq)
 
   ; CHECK: llvm.intr.vaend %[[ALLOCA0]]
   call void @llvm.va_end.p0(ptr %ap)
   ; CHECK: llvm.return
-  ret void
+  ret i32 %ret
 }

mlir/test/Target/LLVMIR/llvmir.mlir

@@ -2275,8 +2275,8 @@ llvm.func @qux(f32)
 
 // CHECK: %struct.va_list = type { ptr }
 
-// CHECK: define void @vararg_function(i32 %{{.*}}, ...)
-llvm.func @vararg_function(%arg0: i32, ...) {
+// CHECK: define i32 @vararg_function(i32 %{{.*}}, ...)
+llvm.func @vararg_function(%arg0: i32, ...) -> i32 {
   %0 = llvm.mlir.constant(1 : i32) : i32
   %1 = llvm.mlir.constant(1 : i32) : i32
   // CHECK: %[[ALLOCA0:.+]] = alloca %struct.va_list, align 8
@@ -2287,12 +2287,14 @@ llvm.func @vararg_function(%arg0: i32, ...) {
   %4 = llvm.alloca %0 x !llvm.ptr {alignment = 8 : i64} : (i32) -> !llvm.ptr
   // CHECK: call void @llvm.va_copy.p0(ptr %[[ALLOCA1]], ptr %[[ALLOCA0]])
   llvm.intr.vacopy %2 to %4 : !llvm.ptr, !llvm.ptr
+  // CHECK: %[[RET:.+]] = va_arg ptr %[[ALLOCA1]], i32
+  %ret = llvm.va_arg %4 : (!llvm.ptr) -> i32
   // CHECK: call void @llvm.va_end.p0(ptr %[[ALLOCA1]])
   // CHECK: call void @llvm.va_end.p0(ptr %[[ALLOCA0]])
   llvm.intr.vaend %4 : !llvm.ptr
   llvm.intr.vaend %2 : !llvm.ptr
-  // CHECK: ret void
-  llvm.return
+  // CHECK: ret i32 %[[RET]]
+  llvm.return %ret : i32
 }
 
 // -----